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17-550

illuminance (at a point of a surface) [Ev; E]

1. quotient of the luminous flux dΦv incident on an element of the surface containing the point, by the area dA of that element

2. equivalent definition: integral, taken over the hemisphere visible from the given point, of the expression LvcosθdΩ, where Lv is the luminance at the given point in the various directions of the incident elementary beams of solid angle dΩ, and θis the angle between any of these beams and the normal to the surface at the given point